linux-kernel - Re: [PATCH] Partially revert "rust: drm: gem: Implement AlwaysRefCounted for all gem objects automatically"

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Message-Id: <E9028A26-2E47-4431-B4B4-C5B416EB36E2@collabora.com>
Date: Thu, 24 Jul 2025 20:13:13 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: Lyude Paul <lyude@...hat.com>,
 nouveau@...ts.freedesktop.org,
 dri-devel@...ts.freedesktop.org,
 rust-for-linux@...r.kernel.org,
 David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>,
 Thomas Zimmermann <tzimmermann@...e.de>,
 Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...il.com>,
 Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <lossin@...nel.org>,
 Andreas Hindborg <a.hindborg@...nel.org>,
 Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>,
 Asahi Lina <lina+kernel@...hilina.net>,
 Alyssa Rosenzweig <alyssa@...enzweig.io>,
 open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Partially revert "rust: drm: gem: Implement
 AlwaysRefCounted for all gem objects automatically"



> On 24 Jul 2025, at 19:27, Danilo Krummrich <dakr@...nel.org> wrote:
> 
> On Thu Jul 24, 2025 at 11:06 PM CEST, Lyude Paul wrote:
>> On Thu, 2025-07-24 at 22:03 +0200, Danilo Krummrich wrote:
>>> On Thu Jul 24, 2025 at 9:15 PM CEST, Lyude Paul wrote:
>>>> -// SAFETY: All gem objects are refcounted.
>>>> -unsafe impl<T: IntoGEMObject> AlwaysRefCounted for T {
>>>> -    fn inc_ref(&self) {
>>>> -        // SAFETY: The existence of a shared reference guarantees that the refcount is non-zero.
>>>> -        unsafe { bindings::drm_gem_object_get(self.as_raw()) };
>>>> -    }
>>>> -
>>>> -    unsafe fn dec_ref(obj: NonNull<Self>) {
>>>> -        // SAFETY: We either hold the only refcount on `obj`, or one of many - meaning that no one
>>>> -        // else could possibly hold a mutable reference to `obj` and thus this immutable reference
>>>> -        // is safe.
>>>> -        let obj = unsafe { obj.as_ref() }.as_raw();
>>>> -
>>>> -        // SAFETY:
>>>> -        // - The safety requirements guarantee that the refcount is non-zero.
>>>> -        // - We hold no references to `obj` now, making it safe for us to potentially deallocate it.
>>>> -        unsafe { bindings::drm_gem_object_put(obj) };
>>>> -    }
>>>> -}
>>> 
>>> IIUC, you'll add rust/kernel/drm/gem/shmem.rs with a new type shmem::Object that
>>> implements IntoGEMObject, right?
>>> 
>>> If this is correct, I think that should work.
>> 
>> Do you mean you think the blanket implementation that we had would work, or
>> that getting rid of it would work?
> 
> The former.
> 
>> Since the blanket implementation we have
>> definitely doesn't compile on my machine once we add more then one
>> IntoGEMObject impl. (before adding it, it works just fine)
> 
> Do you have a branch somewhere, where it doesn't compile?

Hi Lyude, I’m somewhat surprised to be honest. Your gem-shmem code works on
tyr-next, which is currently on top of 6.16-rc2. What exactly doesn’t
compile?

[0] https://gitlab.freedesktop.org/panfrost/linux/-/tree/tyr-next?ref_type=heads